[flang] semantic error for valid volatile uses

[k-arrows]

Test code:
https://github.com/gcc-mirror/gcc/blob/master/gcc/testsuite/gfortran.dg/volatile9.f90

Result (gfortran vs flang-new)

$ gfortran -c volatile9.f90
$

$ flang-new -c volatile9.f90
error: Semantic errors in volatile9.f90
./volatile9.f90:26:18: error: VOLATILE attribute was already specified on 'v13'
       volatile :: v13
                   ^^^

[llvmbot]

@llvm/issue-subscribers-flang-frontend

[klausler]

https://reviews.llvm.org/D150819